The High Acceptance DiElectron Spectrometer HADES 1 is installed at the Helmholtzzentrum fur Schwerionenforschung (GSI) accelerator facility in Darmstadt. It investigates dielectron emission and ...strangeness production in the 1-3 AGeV regime. A recent experiment series focusses on medium-modifications of light vector mesons in cold nuclear matter. In two runs, p+p and p+Nb reactions were investigated at 3.5 GeV beam energy; about 9 times 10 super(9) events have been registered. In contrast to other experiments the high acceptance of the HADES allows for a detailed analysis of electron pairs with low momenta relative to nuclear matter, where modifications of the spectral functions of vector mesons are predicted to be most prominent. Comparing these low momentum electron pairs to the reference measurement in the elementary p+p reaction, we find in fact a strong modification of the spectral distribution in the whole vector meson region.
Recent results obtained with the HADES experimental set-up at GSI are presented with a focus on dielectron production and strangeness in pp and quasi-free np reactions. Perspectives related to the ...very recent experiment using the pion beam at GSI are also discussed.
A
bstract
We report on the measurement of the Central Exclusive Production of charged particle pairs
h
+
h
−
(
h
=
π, K, p
) with the STAR detector at RHIC in proton-proton collisions at
s
= 200 GeV. ...The charged particle pairs produced in the reaction
pp → p
′ +
h
+
h
−
+
p
′ are reconstructed from the tracks in the central detector and identified using the specific energy loss and the time of flight method, while the forward-scattered protons are measured in the Roman Pot system. Exclusivity of the event is guaranteed by requiring the transverse momentum balance of all four final-state particles. Differential cross sections are measured as functions of observables related to the central hadronic final state and to the forward-scattered protons. They are measured in a fiducial region corresponding to the acceptance of the STAR detector and determined by the central particles’ transverse momenta and pseudorapidities as well as by the forward-scattered protons’ momenta. This fiducial region roughly corresponds to the square of the four-momentum transfers at the proton vertices in the range 0
.
04 GeV
2
< −t
1
, −t
2
<
0
.
2 GeV
2
, invariant masses of the charged particle pairs up to a few GeV and pseudorapidities of the centrally-produced hadrons in the range
|η| <
0
.
7. The measured cross sections are compared to phenomenological predictions based on the Double Pomeron Exchange (DPE) model. Structures observed in the mass spectra of
π
+
π
−
and
K
+
K
−
pairs are consistent with the DPE model, while angular distributions of pions suggest a dominant spin-0 contribution to
π
+
π
−
production. For
π
+
π
−
production, the fiducial cross section is extrapolated to the Lorentz-invariant region, which allows decomposition of the invariant mass spectrum into continuum and resonant contributions. The extrapolated cross section is well described by the continuum production and at least three resonances, the
f
0
(980),
f
2
(1270) and
f
0
(1500), with a possible small contribution from the
f
0
(1370). Fits to the extrapolated differential cross section as a function of
t
1
and
t
2
enable extraction of the exponential slope parameters in several bins of the invariant mass of
π
+
π
−
pairs. These parameters are sensitive to the size of the interaction region.
The differential cross section for Z0 production, measured as a function of the boson's transverse momentum (pT), provides important constraints on the evolution of the transverse momentum dependent ...parton distribution functions (TMDs). The transverse single spin asymmetry (TSSA) of the Z0 is sensitive to one of the polarized TMDs, the Sivers function, which is predicted to have the opposite sign in p+p→W/Z+X from that which enters in semi-inclusive deep inelastic scattering. In this Letter, the STAR Collaboration reports the first measurement of the Z0/γ⁎ differential cross section as a function of its pT in p+p collisions at a center-of-mass energy of 510 GeV, together with the Z0/γ⁎ total cross section. We also report the measurement of Z0/γ⁎ TSSA in transversely polarized p+p collisions at 510 GeV.
The differential cross section for Z0 production, measured as a function of the boson’s transverse momentum (pT), provides important constraints on the evolution of the transverse momentum dependent ...parton distribution functions (TMDs). The transverse single spin asymmetry (TSSA) of the Z0 is sensitive to one of the polarized TMDs, the Sivers function, which is predicted to have the opposite sign in p + p → W/Z + X from that which enters in semi-inclusive deep inelastic scattering. In this Letter, the STAR Collaboration reports the first measurement of the Z0/γ* differential cross section as a function of its pT in p+p collisions at a center-of-mass energy of 510 GeV, together with the Z0/γ* total cross section. We also report the measurement of Z0/γ* TSSA in transversely polarized p+p collisions at 510 GeV.
High-precision measurements of flow coefficients
v
n
(
n
=
1
-
4
) for protons, deuterons and tritons relative to the first-order spectator plane have been performed in Au+Au collisions at
s
NN
=
2.4
... GeV with the High-Acceptance Di-Electron Spectrometer (HADES) at the SIS18/GSI. Flow coefficients are studied as a function of transverse momentum
p
t
and rapidity
y
cm
over a large region of phase-space and for several classes of collision centrality. A clear mass hierarchy, as expected by relativistic hydrodynamics, is found for the slope of
v
1
,
d
v
1
/
d
y
′
|
y
′
=
0
where
y
′
is the scaled rapidity, and for
v
2
at mid-rapidity. Scaling with the number of nucleons is observed for the
p
t
dependence of
v
2
and
v
4
at mid-rapidity, which is indicative for nuclear coalescence as the main process responsible for light nuclei formation.
v
2
is found to scale with the initial eccentricity
⟨
ϵ
2
⟩
, while
v
4
scales with
⟨
ϵ
2
⟩
2
and
⟨
ϵ
4
⟩
. The multi-differential high-precision data on
v
1
,
v
2
,
v
3
, and
v
4
provides important constraints on the equation-of-state of compressed baryonic matter.
Density fluctuations near the QCD critical point can be probed via an intermittency analysis in relativistic heavy-ion collisions. We report the first measurement of intermittency in Au+Au collisions ...at $\sqrt{s{NN}}$ = 7.7-200 GeV measured by the STAR experiment at the Relativistic Heavy Ion Collider (RHIC). The scaled factorial moments of identified charged hadrons are analyzed at mid-rapidity and within the transverse momentum phase space. We observe a power-law behavior of scaled factorial moments in Au+Au collisions and a decrease in the extracted scaling exponent (ν) from peripheral to central collisions. The ν is consistent with a constant for different collisions energies in the mid-central (10-40%) collisions. Moreover, the ν in the 0-5% most central Au+Au collisions exhibits a non-monotonic energy dependence that reaches a minimum around $\sqrt{s{NN}}$ = 27 GeV. The physics implications on the QCD phase structure are discussed.
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